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Biology Tools · Updated October 21, 2026 · Reviewed by FormatFlow Editorial

GC Content Calculator

You can paste DNA or RNA sequence, including a FASTA record, and get the GC content with the exact unambiguous-base denominator, base counts, ambiguity codes and sequence length.

Runs in your browserFree · No sign-up · No watermark
DNA/RNA → GC%Runs in your browser
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Everything runs locally in this tab. No file is uploaded while the tool loads.

This tool runs in your tab. Nothing is uploaded and no copy is stored.

How files are handled

At a glance

Works with
DNA/RNA → GC%
Processing
In your browser
Price
Free, no account
Updated
October 21, 2026
All Biology Tools

About GC Content Calculator

GC content is the share of a nucleic acid sequence made up of guanine and cytosine, usually reported as a percentage. It is a quick way to compare sequences, estimate melting behaviour in a general way, and check that a primer or amplicon has the composition a protocol expects.

Sequence data rarely arrives clean. A FASTA file has header lines, raw sequence has line breaks, and ambiguity codes such as N mark positions that were not called. This tool strips the formatting, counts every base, and divides G + C by the unambiguous bases only, showing the numerator, denominator and percentage so ambiguous positions cannot distort the answer.

What this tool does

The details that matter when you use GC Content Calculator for real work.

  • FASTA and raw sequence input

    Paste a single record with its header lines or paste raw sequence; headers, spaces, digits and punctuation are removed before counting.

  • Exact GC denominator

    GC% is (G + C) / (A + T + U + G + C), and the result prints the actual counts so you can check the arithmetic yourself.

  • Ambiguity codes counted separately

    N, R, Y, S, W, K, M, B, D, H, V and X are counted and listed, but they are excluded from the GC fraction.

  • DNA or RNA detection

    The tool reports whether it found thymine, uracil or both, and explains what that means for the sequence.

  • Base table with shares

    Counts and percentage shares for A, T, U, G, C and the ambiguous bases, with a total that matches the parsed sequence length.

  • Copy a summary

    Copy length, base counts, ambiguity codes and the GC percentage as plain text for a notebook or report.

How to use GC Content Calculator

No account, no installation — the whole job happens in this browser tab.
  1. 01

    Paste your sequence

    Type or paste raw sequence or a FASTA record into the box; the analysis starts immediately.

  2. 02

    Let the tool clean the input

    Header lines, whitespace, digits and punctuation are ignored automatically, so a copied file does not need manual tidying.

  3. 03

    Check the detected molecule

    The label says DNA, RNA, mixed or undetermined based on T and U, which tells you how to read the table.

  4. 04

    Read the GC percentage and denominator

    Use the formula line to see exactly which bases were used; ambiguity codes are excluded from both the numerator and the denominator.

  5. 05

    Copy the summary if needed

    Copy summary puts the counts and percentage on the clipboard for a lab notebook or report.

How this tool works

Works with
DNA/RNA → GC%
Processing
Client-side, in your browser. Nothing is uploaded.
Requirements
Chrome, Edge, Firefox or Safari · JavaScript enabled

Limitations worth knowing

  • Ambiguity codes are excluded from GC%, so a sequence containing many Ns can report a percentage over very few unambiguous bases; the denominator is always shown.
  • Several FASTA records pasted together are treated as one combined sequence, so the GC value is for the whole paste rather than each record separately.
  • Very long inputs such as whole genomes are limited by browser memory and may be slow; dedicated command-line tools are better for assemblies.

Privacy and file handling

The sequence you paste stays in this browser tab. Cleaning, counting and the GC calculation run with JavaScript on your device, so nothing is uploaded, stored or logged, and closing the page discards the sequence.

Read the full privacy policy

GC Content Calculator FAQ

Short, direct answers to the questions people ask before using this tool.
How is GC content calculated?
GC content is (G + C) divided by the unambiguous bases (A + T + U + G + C), multiplied by 100. The calculator prints the actual counts and the denominator next to the percentage, so you can see exactly which bases were included.
Why are ambiguous bases excluded from GC content?
An ambiguity code such as N does not identify a base, so counting it as GC or as non-GC would bias the result. The calculator reports ambiguous bases separately and leaves them out of both the numerator and the denominator, which keeps the percentage over known bases only.
Does the calculator accept FASTA files?
Yes. Paste the record text directly. Lines starting with > (the FASTA header) or ; (an older comment marker) are removed, and whitespace, digits and punctuation are ignored. If you paste several records at once their sequences are combined, so analyze one record at a time for per-record GC content.
How does the tool detect DNA versus RNA?
It looks for thymine and uracil: T without U suggests DNA, U without T suggests RNA, both together is reported as mixed, and neither means the molecule cannot be identified from the letters alone. The base table always shows exactly what was counted.
What happens when a sequence has no unambiguous bases?
The length and ambiguity counts are still shown, but GC content is reported as not available, because the denominator would be zero. A warning explains that the codes present are excluded, so a sequence made only of N does not produce a misleading percentage.
What if the pasted text contains letters that are not bases?
Letters that are not IUPAC nucleotide or ambiguity codes, such as E, F, P or Q, are ignored and counted in a warning. Many letters double as ambiguity codes, so check the warning when protein or formatted text is pasted by mistake.
Can I analyze a whole genome with this calculator?
It works for genes, amplicons, primers and other short-to-medium sequences. Whole assemblies can be hundreds of megabytes, which the browser cannot handle comfortably; a command-line tool is the right choice for that scale.
Is the sequence uploaded or stored anywhere?
No. Cleaning, counting and the GC calculation run with JavaScript in this browser tab, so the sequence is never uploaded, stored or logged, and closing the page discards it.

Background reading

Format guides that explain the trade-offs behind GC Content Calculator.